Current versions of FireFTP are compatible with Firefox 21 and later.
You would have to update your current Firefox 20 release or look at an older version of FireFTP.
*https://addons.mozilla.org/en-US/firefox/addon/fireftp/versions/

I have Firefox 23.0.1 but now when I try to use FireFTP I find that all my accounts have disappeared (says choose an account) but the accounts aren't there. I don't know how to get them back and there is no option there to add a new account.

Anyone know how to fix this?

I have Firefox 23.0.1 but now when I try to use FireFTP I find that all my accounts have disappeared (says choose an account) but the accounts aren't there. I don't know how to get them back and there is no option there to add a new account.
Anyone know how to fix this?